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Anders Lund - One of the best experts on this subject based on the ideXlab platform.
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structure of dimeric radical cations of benzene and toluene in halocarbon matrices an epr endor and mo study
Physical Chemistry Chemical Physics, 2000Co-Authors: Yoshiteru Itagaki, Nikolas P Benetis, Ramakant M Kadam, Anders LundAbstract:The structure of dimeric cations of benzene and toluene formed in X-irradiated halocarbon matrices containing relatively high concentration of the solutes was investigated. EPR and ENDOR (electron nuclear double resonance) spectra of these dimer cations were observed and accurate values of the hf Coupling constants were obtained. The ENDOR spectrum of the dimeric radical cation of benzene, (C6H6)2+, exhibited hf Couplings due to twelve equivalent protons and the Isotropic Coupling (aiso) was almost one-half of that in the monomer cation. ENDOR transitions with a rhombic symmetry were observed in a CFCl3 matrix, whereas clear axially symmetric transitions with ∣A‖∣<∣A⊥∣ were obtained in CF3CCl3 even at 50 K. The rhombic dipolar Coupling tensor was used as a parameter to evaluate the distance between the two partner rings. As regards the toluene dimer cation, (CH3C6H5)2+, ENDOR transitions of the CH3 and H(4,4′) protons were observed. The Isotropic hf Coupling of the CH3 protons deviated even more strongly from the half value, being rather close to one-third of the value of the monomer. The hf Coupling of the H(4,4′) protons was almost half the Coupling of the monomer. It was suggested that the anomalous hf Couplings of the CH3 protons were due to the interaction between the two rings through the σ-bond. Density functional theory (DFT) calculations were employed to obtain the optimized geometries and hf Coupling tensors and suggested sandwich structures in both dimers. Furthermore, the distances between the two rings and the anomalous CH3 protons hf Coupling in the (CH3C6H5)2+ were successfully evaluated.
Sergei A. Dikanov - One of the best experts on this subject based on the ideXlab platform.
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Hyperfine and Nuclear Quadrupole Tensors of Nitrogen Donors in the QA Site of Bacterial Reaction Centers: Correlation of the Histidine Nδ Tensors with Hydrogen Bond Strength
2016Co-Authors: Er T. Taguchi, Patrick J. O’malley, Colin A Wraight, Sergei A. DikanovAbstract:ABSTRACT: X- and Q-band pulsed EPR spectroscopy was applied to study the interaction of the QA site semiquinone (SQA) with nitrogens from the local protein environment in natural abundance 14N and in 15N uniformly labeled photosynthetic reaction centers of Rhodobacter sphaeroides. The hyperfine and nuclear quadrupole tensors for His-M219 Nδ and Ala-M260 peptide nitrogen (Np) were estimated through simultaneous simulation of the Q-band 15N Davies ENDOR, X- and Q-band 14,15N HYSCORE, and X-band 14N three-pulse ESEEM spectra, with support from DFT calculations. The hyperfine Coupling constants were found to be a(14N) = 2.3 MHz, T = 0.3 MHz for His-M219 Nδ and a( 14N) = 2.6 MHz, T = 0.3 MHz for Ala-M260 Np. Despite that His-M219 Nδ is established as the stronger of the two H-bond donors, Ala-M260 Np is found to have the larger value of a(14N). The nuclear quadrupole Coupling constants were estimated as e2Qq/4h = 0.38 MHz, η = 0.97 and e2Qq/4h = 0.74 MHz, η = 0.59 for His-M219 Nδ and Ala-M260 Np, respectively. An analysis of the available data on nuclear quadrupole tensors for imidazole nitrogens found in semiquinone-binding proteins and copper complexes reveals these systems share similar electron occupancies of the protonated nitrogen orbitals. By applying the Townes−Dailey model, developed previously for copper complexes, to the semiquinones, we find the asymmetry parameter η to be a sensitive probe of the histidine Nδ−semiquinone hydrogen bond strength. This is supported by a strong correlation observed between η and the Isotropic Coupling constant a(14N) and is consistent with previous computational works and our own semiquinone-histidine model calculations. The empirical relationship presented here for a(14N) and η will provide an important structural characterization tool in future studies of semiquinone-binding proteins
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Hyperfine and Nuclear Quadrupole Tensors of Nitrogen Donors in the QA Site of Bacterial Reaction Centers: Correlation of the Histidine Nδ Tensors with Hydrogen Bond Strength
2015Co-Authors: Alexander T. Taguchi, Patrick J. O’malley, Colin A Wraight, Sergei A. DikanovAbstract:X- and Q-band pulsed EPR spectroscopy was applied to study the interaction of the QA site semiquinone (SQA) with nitrogens from the local protein environment in natural abundance 14N and in 15N uniformly labeled photosynthetic reaction centers of Rhodobacter sphaeroides. The hyperfine and nuclear quadrupole tensors for His-M219 Nδ and Ala-M260 peptide nitrogen (Np) were estimated through simultaneous simulation of the Q-band 15N Davies ENDOR, X- and Q-band 14,15N HYSCORE, and X-band 14N three-pulse ESEEM spectra, with support from DFT calculations. The hyperfine Coupling constants were found to be a(14N) = 2.3 MHz, T = 0.3 MHz for His-M219 Nδ and a(14N) = 2.6 MHz, T = 0.3 MHz for Ala-M260 Np. Despite that His-M219 Nδ is established as the stronger of the two H-bond donors, Ala-M260 Np is found to have the larger value of a(14N). The nuclear quadrupole Coupling constants were estimated as e2Qq/4h = 0.38 MHz, η = 0.97 and e2Qq/4h = 0.74 MHz, η = 0.59 for His-M219 Nδ and Ala-M260 Np, respectively. An analysis of the available data on nuclear quadrupole tensors for imidazole nitrogens found in semiquinone-binding proteins and copper complexes reveals these systems share similar electron occupancies of the protonated nitrogen orbitals. By applying the Townes–Dailey model, developed previously for copper complexes, to the semiquinones, we find the asymmetry parameter η to be a sensitive probe of the histidine Nδ–semiquinone hydrogen bond strength. This is supported by a strong correlation observed between η and the Isotropic Coupling constant a(14N) and is consistent with previous computational works and our own semiquinone-histidine model calculations. The empirical relationship presented here for a(14N) and η will provide an important structural characterization tool in future studies of semiquinone-binding proteins
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Hyperfine Interaction Tensors of ^13C Nuclei for Ring Carbons of Ubisemiquinone-10 Hydrogen Bonded in Alcohol Solvents
Applied Magnetic Resonance, 2014Co-Authors: Rimma I. Samoilova, Alexander T. Taguchi, Patrick J. O’malley, Sergei A. Dikanov, Johan LugtenburgAbstract:The anion radicals of ubiquinones-10 ^13C chemically labeled at the C_5 or C_6 ring positions in alcohol have been studied by 1D and 2D ESEEM to define the hyperfine interaction tensors with the ^13C nuclei. Analysis of the cross-peak line shapes and simulations of the spectra allowed us to conclude that the hyperfine tensors are characterized by an anIsotropic component T ~6 MHz and an Isotropic Coupling a ~−3 MHz with support from DFT calculations. However, these values were found to be inconsistent with the shift of the sum combination harmonic in the four-pulse ESEEM spectra. Simulations resolve this apparent discrepancy by showing that the shift of the sum combination to lower frequency and its broadening can be accounted for by a distribution of the hyperfine Couplings. A spread of the methoxy group conformations, as supported by previous experimental observations, is suggested as the mechanism influencing the distribution of the hyperfine Couplings for the ring carbons.
Jochen Autschbach - One of the best experts on this subject based on the ideXlab platform.
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two component relativistic hybrid density functional computations of nuclear spin spin Coupling tensors using slater type basis sets and density fitting techniques
Journal of Chemical Physics, 2008Co-Authors: Jochen AutschbachAbstract:Computations of indirect nuclear spin-spin Coupling constants using two-component relativistic density functional theory with a hybrid functional are reported. The program implementation makes use of a Slater-type orbital expansion of the molecular orbitals and the zeroth-order regular approximation for the treatment of relativistic effects. Exact exchange terms in the Kohn–Sham response kernel were computed using a fitting procedure. Computations with the PBE0 hybrid functional were carried out for heavy-atom-ligand-one-bond Couplings in PbH4, Pb(CH3)2H2, Pb(CH3)3H, three platinum complexes, the interhalogen diatomics such as ClF, ClBr, ClI, BrF, BrI, IF, and the series Tl-X with X=F, Cl, Br, I. The hybrid functional computations performed very well. In particular, for the Isotropic Coupling and the Coupling anisotropy of Tl-X, the PBE0 hybrid functional yielded considerably improved agreement with experiment.
Yoshiteru Itagaki - One of the best experts on this subject based on the ideXlab platform.
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structure of dimeric radical cations of benzene and toluene in halocarbon matrices an epr endor and mo study
Physical Chemistry Chemical Physics, 2000Co-Authors: Yoshiteru Itagaki, Nikolas P Benetis, Ramakant M Kadam, Anders LundAbstract:The structure of dimeric cations of benzene and toluene formed in X-irradiated halocarbon matrices containing relatively high concentration of the solutes was investigated. EPR and ENDOR (electron nuclear double resonance) spectra of these dimer cations were observed and accurate values of the hf Coupling constants were obtained. The ENDOR spectrum of the dimeric radical cation of benzene, (C6H6)2+, exhibited hf Couplings due to twelve equivalent protons and the Isotropic Coupling (aiso) was almost one-half of that in the monomer cation. ENDOR transitions with a rhombic symmetry were observed in a CFCl3 matrix, whereas clear axially symmetric transitions with ∣A‖∣<∣A⊥∣ were obtained in CF3CCl3 even at 50 K. The rhombic dipolar Coupling tensor was used as a parameter to evaluate the distance between the two partner rings. As regards the toluene dimer cation, (CH3C6H5)2+, ENDOR transitions of the CH3 and H(4,4′) protons were observed. The Isotropic hf Coupling of the CH3 protons deviated even more strongly from the half value, being rather close to one-third of the value of the monomer. The hf Coupling of the H(4,4′) protons was almost half the Coupling of the monomer. It was suggested that the anomalous hf Couplings of the CH3 protons were due to the interaction between the two rings through the σ-bond. Density functional theory (DFT) calculations were employed to obtain the optimized geometries and hf Coupling tensors and suggested sandwich structures in both dimers. Furthermore, the distances between the two rings and the anomalous CH3 protons hf Coupling in the (CH3C6H5)2+ were successfully evaluated.
W Lubits - One of the best experts on this subject based on the ideXlab platform.
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orientation selected endor of the active center in chromatium vinosum nife hydrogenase in the oxidized ready state
Journal of Biological Inorganic Chemistry, 1999Co-Authors: C Gesner, Matthias Stein, Simon P J Albracht, W LubitsAbstract:Electron nuclear double resonance (ENDOR) was applied to study the active site of the oxidized "ready" state, Nir, in the [NiFe] hydrogenase of Chromatium vinosum. The magnetic field dependence of the EPR was used to select specific subsets of molecules contributing to the ENDOR response by stepping through the EPR envelope. Three hyperfine Couplings could be clearly followed over the complete field range. Two protons, H1 and H2, display a very similar large Isotropic Coupling of 12.5 and 12.6 MHz, respectively. Their dipolar Coupling is small (2.1 and 1.4 MHz, respectively). A third proton, H3, exhibits a small Isotropic Coupling of 0.5 MHz and a larger anIsotropic contribution of 3.5 MHz. Based on a comparison with structural data obtained from X-ray crystallography of single crystals of hydrogenases from Desulfovibrio gigas and D. vulgaris and the known g-tensor orientation of Nir, an assignment of the 1H hyperfine Couplings could be achieved. H1 and H2 were assigned to the β-CH2 protons of the bridging cysteine Cys533 and H3 could belong to a β-CH2 proton of Cys68 or to a protonated cysteine (-SH) of Cys68 or Cys530.